Articles such as food items such as specifically meat, poultry and seafood products are often packaged in thermoplastic films or laminates in order to protect the product to be packaged from exterior abuse and environmental contamination, and to provide a convenient and durable package for distribution of the product and display in a display case or other point of sale. For several years, the food packaging industry has also used packaging films from which bags and casings have been made that are of improved structural soundness such that they have been fully characterized as “cook-in.” These packages allow for production of a precooked food product that has been attractively packaged using a film within which the product was precooked. The films have been used primarily for cook-and-ship end-use, wherein a meat product is packaged in the film and cooked while in the film, and is thereafter cooled and shipped to wholesaler, retailer, or consumer, without being over wrapped and without having the film stripped therefrom and thereafter being re-packaged in another film.
Cook-in packaged foods are essentially pre-packaged, pre-cooked foods that may be directly transferred to the consumer in this form. Cook-in packaging materials maintain seal integrity, i.e., any heat-sealed seams should resist being pulled apart during cook-in, and are typically conformable to the contained food product. For example, conformability is achieved by the film being heat shrinkable so as to form a tightly fitting package. Additional desirable characteristics of films for use in cook-in applications include delamination-resistance, low O2-permeability, heat-shrinkability, and optical clarity. When dealing with meat products in particular, it is usually desirable to provide a film having good oxygen barrier characteristics, in order to reduce the passage of oxygen through the film so that detrimental effects on freshness, color, and other properties of the meat product are minimized.
Cook-in films preferably have a tendency for adhesion to the food product thereby preventing “cook-out”, i.e., “purge”. This meat adhesion characteristic of the meat contact layer serves to both prevent cook-out (and thereby prevent an unattractive package or the need to strip the film from the meat and repackage the meat), to increase product yield, and to prevent the meat from losing juices which improve its edible character. The term “adhere”, with respect to meat-adhesion, refers to a meat-contact surface of the film which bonds during cook-in to the contained food product to an extent sufficient to substantially prevent accumulation of fluids between the film and the contained meat product. See, e.g., U.S. Pat. No. 6,346,285 and U.S. Patent Application 2002/0160129.
It is desirable to provide a package for foodstuffs comprising a film with increased heat resistance, allowing for heating at higher temperatures while maintaining seal quality and barrier properties. Such packages are suitable for heating or cooking in conventional, convection or microwave ovens (“dual-ovenable”) or for heating in boiling water. The convenience of being able to purchase a fresh, partially cooked or value-added foodstuff in a package at the retail level and be able to insert the package directly into a conventional oven or microwave and cook the foodstuff in the package is highly desirable.
Polyethylene terephthalate (PET) films have high heat resistance, making them suitable for cook-in and ovenable uses. Such films provide strength and protection, though they typically have the drawback that they are sealed with an adhesive since polyester is not heat-sealable except at exceptionally high temperatures. Packages (such as bags) sealed with adhesive are generally not as strong in the seal area as heat-sealed packages, and cannot be closed on the open end by existing heat-seal equipment. It is desirable to provide a package with high heat resistance and low O2-permeability that can be heat sealed by existing heat-seal equipment. Other desirable attributes include puncture-resistance from internal or external puncture sources, ability to withstand freezer temperatures of −40° F., heat-shrinkability, ability to be self-venting (venting during cooking by temperature dependent processes), and optical clarity. These attributes can be achieved by using coextruded multilayer film structures having polyester-based copolymers. See, e.g., Japanese Patents JP2663578B2, JP4051423B, and JP5043584B.